SteamPunk is a sub-genre of speculative fiction, basically a “what if” genre in which geniuses are able to produce mechanical wonders such as robots and floating battle ships that run on steam power and sometimes clock work. The genre is modeled after CyberPunk and usually has some sort of anti-establishment theme, but is usually set in the Victorian era or the Old West.
To get a feel for the genre and hopefully generate a little more interest, since it is an obscure, although very interesting genre, I've decided to create a little list of the themes, settings, and plot devices that work well, or could work well, for SteamPunk fiction.

Alternate Universe- Technically SteamPunk is always an alternate universe, but some SteamPunk writers create unique worlds with it's own countries, races, and animals. Much like most other fantasy fiction, except with a lot of steam technology.

Anime, Manga and Comics- Anime, Manga, and Comic books often use speculative steam technology especially in Victorian or fantasy settings.(10)They mix with anime, manga, and comic tropes like superheroes and cat-girls.

Antiquities-Victorian items like eye glasses and pocket watches really set the mood for a steam punk story and give you a little information about the character who owns them.

Balloons- Hot air balloons and zeppelins. A means of flying, used in some way in most SteamPunk fiction that features massive airships or vast fleets of flying machines.

Body Art- Piercings and tattoos.(1) Mostly a modern thing in the west, but it goes well with the “punk” part of SteamPunk, meaning pro-self expression and anti-establishment, and the genre already takes artistic liberties with history because of the fantastic inventions of the stories.

Body Modification- Mechanical body parts may be thought of as mostly a modern and futuristic sci-fi concept, but SteamPunk stories often feature characters with artificial body parts. These can be simple bone or skin replacements, like the character with an iron cranium on the movie “Wild Wild West”,(2) or movable claws and limbs.

Bombs- Gunpowder, nitroglycerin, and sometimes TNT. They can work on fuses, impact detonation, or retro clock work timers to strike a spark on a fuse or shake up a vial of nitroglycerin.

Cannons- Cannons have been around since before steam. Fictional super cannons may appear in a story, such as the cannon that shot a man to the moon in Jules Verne's “From the Earth to the Moon.”(3)
Cannons can often be found in flying fortresses to fend of pirates in SteamPunk.
Cannons may be sleek and professional or they may be something that some guy built in his shop that shoots garbage.

Chemistry- Modern chemistry came about in the beginning of the Victorian era. The idea of turning lead into gold was more or less over except for in the fringe of the community, but the science was still new enough for some imaginative mutation and immortality potion stories.(4)

City States- City States are possible in times of war and chaos, easily envisioned in SteamPunk, or they may be the result of a city leader becoming so powerful, in terms of charisma, science, and military, that the city is actually able to declare independence from a nation. Such a thing is often the goal of super villains in comic books and cartoons. “Today Metropolis, tomorrow the world!”

Class Divide- The oldest and most intense conflict is the conflict between rich and poor. Workers in the Victorian era had it pretty rough, working in dangerous conditions with low pay,(32) and anarchists and socialists sometimes committed acts of indiscriminate violence against the rich upper classes.

Clock Work-Quite a bit older than steam technology. Clock work sometimes stands alone in its own ClockPunk genre, usually set during the Renaissance and inspired primarily by Leonardo da Vinci.
Clock work is a necessity in SteamPunk and is interchangeable with steam technology. A robot might have a steam engine or a wind up key, depending on the writers preference.

Colonialism- Colonialism was presented mostly as a positive thing in most of the early adventure novels that SteamPunk draws from.(5) If this is brought up in SteamPunk it is usually in parody or with the expansionists as the villains lifted up as heroes by society.

Detective-Sherlock Holmes was brought into fiction in the late 19th century.(6) Detective stories of this kind tended to be highly educational and downplayed the violent parts.

Difference Engines- Or Analytical Engines. Difference engines were mechanical calculators designed to calculate complex polynomials. The earliest designers were in the eighteenth century but the device was not actually made until the nineteenth century. SteamPunk fiction uses the Difference Engine as an early form of computer.(7)

Dystopia- The Victorian era had a lot of social, economic and political problems, hidden by adventure novelists of the past but highlighted by modern political SteamPunk writers who have things to say about society, usually as a thinly veiled parody.(8) See “Subversiveness.”

Explosions- For comedic effect, like a cartoon bomb blowing up and leaving soot on someone's face, or more serious, like nitroglycerin explosions.

Fantasy- See “Alternate Universes.”

Flying Cities- Flying Cities in SteamPunk are usually military in nature, flying fortresses with a fleet of military flying machines.(9) Flying cities and fortresses are usually blimps or zeppelins, but more fantastic stories may have massive colonies suspended in the air by constantly rotating helicopter blades.

Flying Machines- These can be blimps or hot air balloons, something resembling modern planes, rockets, gliders, clockwork helicopters, or machines that flap wings like birds to achieve lift.(11)

France- The wars, class divide, religious issues and politics of Europe from the French Revolution through World War I are perfect settings for political SteamPunk.

Gears- The most important thing in the world for the SteamPunk genre. Steam and clockwork machines can't work without them and they provide other plot devices, for instance a final showdown between the hero and villain in the bowels of some gargantuan machine, going at each other while trying to avoid the giant crushing gears.

Germany- The Industrial Machine. Germany is well known for innovation and work ethic, especially pre-WWI Germany, and has a long history of conflict with other European countries and internal political strife.

Giant Machines- Towering titans blotting out the sun with smoke. Typically the invention of a mad scientist, a secret society, or countries at war.

Gunpowder-A technology available at the time. Used for weapons, bombs, or fuel for rockets.

Grease- A small touch, greasy hands and clothes for the tinkers, but it's so necessary with SteamPunk fiction that it deserves it's own place in the list.

Industrial MegaCity-Claustrophobic with huge gears and plums of smoke and steam. Representative of an industrial dystopia.

Ironclads-Those metal battleships that got their start in the American Civil War. They inspired war fiction of the period and are common in modern SteamPunk as locations for a story or weapons of war.

Little Gizmos- Cute little mascots made from old broken clocks or sinister swarms of predatory metallic insects. The smaller machines tend to be clockwork instead of steam.

Mad Scientist- The classic super villain. This sort of character has a wide range of possibilities in SteamPunk. The Mad Scientist may heartlessly perform nightmarish experiments in pursuit of his or her goals,(13) often something like immortality or world domination, or the mad scientist could be a hero, creating helpful machines for those in need while harboring a few megalomaniac tendencies. This last sort is common in manga and anime.

Mars- As recently as the early twentieth century the idea of intelligent life on other planets in our solar system was strange but not thought to be impossible. See Unknown.
Verne and Wells both imagined fantastic monsters on or from nearby worlds.(3)(24)

Magnetism- There are a lot of things one can do with magnets, especially when one adds the artistic liberties of pseudoscience.

Magnified Light- Like burning ants under a magnifying glass. This idea can be taken to a ridicules degree in SteamPunk with solar powered light cannons or heat rays.

Magnifying Glass-Used to magnify light, search for clues, or can be added to goggles and give an engineer a free hand.

Mech Suit- More of a futuristic cyber anime concept, but a mech suit in a SteamPunk story would probably be big and slow, requiring room for the steam engine, levers, and gears.
Mech Suits in SteamPunk would probably be built to help in heavy lifting and hostile environment work, although guns could reasonably be added and a rocket could give a mech suit the ability to fly.(14)

Microscopes-Microscopes have existed since the 1600s. They are a must have for any mad scientist, chemist, or biologist.

Mirrors- A well known tool of illusionists. Reflects light and images and number of mirrors positioned in just the right way can make an image appear 3-dimensional.

Monsters- More on the fantasy side of SteamPunk. Monsters might mean using the antiquated sciences of the nineteenth and early twentieth centuries or giving no explanation for the existence of a monster at all.

Neo-Victorianism-More of a form of modern goth fashion, but the mood works well in SteamPunk stories, role playing games, fashion and cosplay.(15)

News Papers- See “Print Media.”

Nitroglycerin- Nitroglycerin in SteamPunk, as with most inventions, can appear in any decade the story might be set in, but the process of creating pure Nitroglycerin was perfected in the 1860s by Alfred Nobel and a series of tragic explosions soon followed.(16)
These nasty explosives are popular in SteamPunk Westerns as well as regular westerns. There are two reasons for this. First, it is an early technology, perfected not long after the end of the American Civil War, and the second is that the pure form of nitroglycerin is just as dangerous and combustible as it is presented in even the most over dramatic SteamPunk Westerns, able to level buildings and blow trains apart like wet paper.

Nobility- Class was very important in the Victorian era. Nobles in Victorian England were even less likely to get in trouble with the law than rich people today. Nobles in SteamPunk can be heroes, using their wealth and education for the benefit of the lower class or arrogant villains, doing for themselves at the expense of the poor.

Old Technology- Any old idea used in an imaginative or fantastic way, like zeppelins with cannons or insanely complex difference engines.

Pirates- Especially air pirates.(17) Pirates in SteamPunk function mostly the same as pirates in any other fiction. Villains, anti-heroes, or heroic rebels with a few personality flaws.

Political Parties- Usually fictional or extinct political parties. Political parties tend to be the bad guys in SteamPunk.

Politics-Conflicts surrounding votes, like voter fraud or propaganda, or government policies, usually related to the class divide or taxes.

Pollution- Burning coal causes smog and ozone depletion (if you believe the theories regarding that). In a fictional world where basically everything runs on steam power, pollution would be an excellent conflict or side conflict. Realistic, such as species extinction and ocean pollution, or more fantastic conflicts with mutants and monsters.

Print Media- “Extra, extra, read all about it!” SteamPunk stories usually do not feature television sets or even radio, so most media in the stories are books, pamphlets, and newspapers.
Newspaper media also meshes well with political subversiveness in SteamPunk stories. Several lines in The Declaration of Independence were taken from Thomas Paine's political pamphlet “Common Sense”(18) and a sensationalist article by William Randolf Hearst accusing the Spanish of bombing the USS Maine was the most direct cause of the Spanish American War.(19)

Pseudoscience- Old or persisting theories with little or no supporting science and usually a ton of science that contradicts the theory or proves it completely wrong.
An example is the Hollow Earth theory(12) of the 19'th century, still common in comic books and retro-futurist literature. People once imagined grand cities and even dinosaurs living under the Earth's crust.(21)

Realism vs Fantasy- SteamPunk, like all science fiction, can be divided between stories that try to be realistic with the ideas and inventions in the stories and those that ask the reader to “just go with it.” Comics tend to “just go with it” while novels, novellas, and even short stories have a bit more room to explain why something works.
Of course there's only so much one can do with steam power and springs, but a creative person can actually convince a person that a determined and brilliant person can design a difference engine advanced enough to rival early computers.(7)
Both “hard” SteamPunk and “Fantastic” SteamPunk have their strengths and weaknesses, but an avid reader can be a fan of both, depending on the mood, and not feel ashamed.

Retro-Futurism- This means looking at the pasts idea of the future, either in fiction or educated predictions, and making a story about it. These can be done for comedy or nostalgia.
To be considered true Retro-Futurism a story must keep to the future expectations of the time used as reference. That means abandoning many favorite SteamPunk themes such as steam computers.(20)

Rockets- More like bottle rockets than modern gas propelled rockets. SteamPunk fiction uses rockets as military equipment and high speed transportation,(9) either flying through the air or attached to a land vehicle.

Robots- These can be simple automations of gears and springs with pre-programmed music or unexplainable, autonomous hulks with fully functional voice and language. See “Fantasy vs Realism.”
Robots are big in SteamPunk, even though we haven't quite gotten the hang of that one even with electricity, computers and advanced AI.
Robots in SteamPunk range from little clockwork toys to massive engines of war bellowing smog.

Schools- Universities have always been centers of political change and strife, and a protagonist has to learn to make that giant robot from somewhere. Boarding schools and charm schools are also very Victorian and where better to fine a professor role model/father figure than at a University? In a nut shell, great SteamPunk setting.

Secret Societies- Back in the eighteenth and nineteenth centuries the idea of Secret Societies was very scary and not just for conspiracy theorists. There was actually an “Anti-Masons” political party formed in 1826 in response to the disappearance of a self professed member of the Free Masons who disappeared after promising to publish a book exposing the secrets of the secret society.(22)
Secret Society plots fit with the political subversive side of SteamPunk. Secret Societies are thought of as run by the rich and politically elite making them fun villains, but Secret Societies can also be the heroes of a SteamPunk story, opposing a repressive government.

Social Mores- Although SteamPunk tends to focus more on politics and class divide, social rules are made to be broken in all Punk fiction. This could mean, just for an E rated example, a peasant marrying a noble, or something more edgy even for modern times.

Space Travel- People have always dreamed of the possibilities, and horrors, of space travel. Jules Verne imagined a giant cannon that would blast the insane astronaut to the moon(3) and H.G Wells “War of the Worlds” is famous for bringing squid like invaders from Mars to Earth.(24)

Submarines- The first American Submarine was invented during the civil war. The small object was designed to get under English ships and drill holes in the bottom, but none of the attempts succeeded.(23)
Submarines in SteamPunk fiction are yet another reason to thank the great Jules Verne. In “20,000 Leagues Under the Sea” Verne's character Nemo designed a massive submarine to explore the ocean depths and survive giant squid attacks.(30)
Submarines in SteamPunk fiction either deal past speculations of the unknown or the technical problems of relying completely on a machine to protect ones life in an ocean environment.

Subversiveness- Political and Social. While CyberPunk tends to focus on dystopian future societies ruled by big business, SteamPunk, when it is has a political message which it often does, highlights old fashioned social taboos, repressive governments, and especially economic class divide. Firestorm issues in the Victorian era and still touchy today.(1)

Sword Fights- It is a lot easier to find an excuse for a sword fight in SteamPunk fiction than in most fiction set in present or future times. In the Victorian era gentlemen still dueled each other with well laid out rules and if a pistol is out of reach or out of ammunition

Tanks- Or “Land Ironclads.” Based on the novella by H.G Wells.(25)

Telescopes- Hans Lipperhey and Jacob Metius separately created the first known designs for a telescope and failed to get patents in 1608.(26) In the same century Galileo designed a telescope of his own and discovered four moons of Jupiter.(27)
In SteamPunk fiction telescopes can be portable accessories or massive constructions moved around by gears and levers.

Theater- A gathering for the elite; Also a stage for subversive art in the Victorian era, particularly the cheaper “bad part of town” theaters.
Great settings for taboo breaking as art has always gotten a lot of flak.

Tinker's Workshop-A likely setting in a story with a tinker or inventor as the protagonist. Tinker's Workshops in SteamPunk fiction are typically cluttered, dusty, greasy and littered with gears and springs.

Trains- The importance of trains for the SteamPunk genre can not be over stated. Trains were the first form of mass transportation, connecting whole continents and diffusing culture and information.
In SteamPunk fiction trains serve as mobile bases as in Wild Wild West,(2) a means of long range transportation, especially in a SteamPunk Western, and a dramatic, if overused scene for the final showdown between the hero and the villain.

Underground- As in politically. In SteamPunk, the protagonist often joins anti-government groups or is involved in activities that are banned by the fictional government or society.

Under Water- Even today there is a lot we don't know about the ocean depths, but in the Victorian era it was easy to imagine horrible sea creatures and lost cities populated by fish people.
Underwater SteamPunk , is inspired by Jules Verne, a founding father of SteamPunk as a whole. The comic and movie “The League of Extraordinary Gentlemen” features Captain Nemo and his submarine from Verne's book “20,000 Leagues Under the Sea”(30) and the same character abducts sailors in the 1964 movie “Captain Nemo and the Underwater City.”(29)

Unknown- SteamPunk stories are usually set in a world or time where people still imagined horrible monsters in the depths of the sea, ethereal flying dragons in the outer solar system, or civilizations of mole men under ground. Old ideas like the Hollow Earth Theory(12) inspired scores of “center of the Earth” adventures that continue today even now that we know that the inside of the Earth is molten.
SteamPunk writers often take artistic liberties with todays science and present these old ideas as fact in their fictional worlds.

Verne and Wells- Two of the founding fathers of all science fiction. Their novels and stories were written in time periods that SteamPunk stories are typically set in, the early nineteenth and early twentieth centuries, giving them a slightly different perspective than SteamPunk writers. Verne and Wells envisioned things that were not known to be impossible at the, such as complicated surgery on animals to make them able to speak and walk around like human beings in Wells' “The Island of Doctor Moreau,”(30) and other things that predicted future technology such as in Verne's “20,000 Leagues Under the Sea.”
SteamPunk draws heavily on the works of Jules Verne and H.G Wells in terms of fantastic inventions and adventures.(30)

Wonder Kid- A Wonder Kid character is a child genius, similar to characters in anime. A Wonder Kid is able to create fantastic things, usually with few resources.
In the '60s and '70s these characters tended to be male and the more common name for this theme is “Boy Genius,” like Tom Swift, the adolescent main character of the novel series named after him, who created motorcycles and flying machines.(31)
Times are changing though, and you now see more female Wonder Kids, especially in comics and anime like the online comic “Girl Genius” who's main character Agatha Clay creates steam powered and clock work machines in her sleep.(10)